Cerebral ischemia of 30 minutes duration significantly increases the level of HVA, while it has no influence on MOPEG-SO4 and 5-HIAA levels. MOPEG-SO4 level was not changed 1 hour after onset of ischemia, while that of 5-HIAA was increased. However, MOPEG-SO4 level was increased after 2 hours of ischemia. It took about 2 hours for catecholamine metabolites to reach the highest values, and 1 hour for 5-HIAA. Thereafter, these levels remained unchanged. In gerbils' brains, inhibitor of monoamine oxidase, pargyline, significantly decreased the levels of HVA, MOPEG-SO4 and 5-HIAA. This decrease could be prevented by: (1) blockage of active out-transport of metabolites from the brain of CSF by means of probenecid or (2) by the exposure of the animals to ischemia. It appears that ischemia has the same influence on pargyline-treated animals as probenecid.